The invention relates to an injection valve for a common rail injection system.
In the case of a common rail injection system, the fuel is injected into the combustion chamber of an internal combustion engine at a pressure of up to 2000 bar. The high fuel pressure requires precise control of the injection time and of the injection quantity. Furthermore, it is necessary, for internal combustion engines which are operated with diesel fuel, to carry out an exact pre-injection with a small quantity of fuel in order to minimize the noise of the internal combustion engine and also the emission of pollutants. For the abovementioned reasons, it is necessary to coordinate the injection valve very precisely, so that an optimum shaping of the injection profile is achieved.
The article xe2x80x9cA Common Rail Injection System For High Speed Direct Injection Diesel Enginesxe2x80x9d, SAE paper 980803, by N. Guerrassi et al. discloses a fuel injection valve for a common rail injection system which has a control chamber which is supplied with fuel by a fuel line via a inlet throttle. The control chamber is connected via a outlet throttle to a outlet line which can be connected to a fuel reservoir via an electromagnetic valve. Furthermore, a bypass throttle is provided which creates a connection between the fuel line and the outlet line. The control chamber is bounded by a nozzle needle which is arranged in an axially movable manner in a nozzle body. The nozzle needle is guided through a nozzle chamber which is connected to the fuel line. Furthermore, the nozzle needle has pressure surfaces which are acted upon by the fuel pressure prevailing in the nozzle chamber and apply force to the nozzle needle in the direction of the control chamber. A nozzle spring which prestresses the nozzle needle in the direction of its sealing seat is provided in the control chamber. The pressure in the pressure chamber is controlled as a function of the opening position of the electromagnetic valve. If the valve is opened, fuel flows out of the pressure chamber via the outlet throttle and at the same time less fuel flows in via the inlet throttle, so that the pressure in the control chamber drops. As a consequence of this, the nozzle needle is moved in the direction of the nozzle chamber, the nozzle needle lifting with its point off a sealing seat and releasing a connection between the fuel line and injection holes.
If the electromagnetic valve is now closed, then fuel flows into the control chamber via the inlet throttle, via the bypass throttle and the outlet throttle. In this manner, the pressure in the control chamber is rapidly increased, so that the nozzle needle is pressed relatively rapidly onto its sealing seat in the nozzle body and the injection is therefore rapidly ended.
The injection valve described has the disadvantage of the nozzle spring being situated in the control chamber and hence a relatively large control chamber being necessary, which constitutes a large harmful volume. Furthermore, the installation of the nozzle spring in the control chamber gives rise to the risk of, during installation, particles of dirt entering into the control chamber and collecting in the outlet throttle and impairing the functioning capability of the injection valve. Cavitation bubbles arising in the inlet throttle may damage the nozzle spring.
The object of the invention is to provide an injection valve with a simpler construction, in which the functioning of the hydraulic control system is not impaired.
The object of the invention is achieved by an injection valve comprising:
a fuel line which is guided to a control chamber via a inlet throttle,
a outlet throttle which connects a return line to the control chamber,
a control valve which is connected in the return line upstream of a return flow,
a bypass throttle which connects the fuel line to the return line,
a nozzle needle which is arranged movably in a nozzle chamber, wherein the nozzle chamber being connected to the fuel line, the nozzle needle being connected to a control piston, the control piston bounding the control chamber, part of the return line is designed as a valve chamber, and the bypass throttle opens into the valve chamber.
A method of operating an injection valve comprises the steps of:
storing fuel at high pressure in a fuel line;
supplying the high pressured fuel to a valve chamber, to a control chamber for controlling a nozzle needle;
controlling the pressure in the control chamber through a servo valve and an outlet throttle coupling the valve chamber and the control chamber.
Part of the return line is preferably designed as a valve chamber into which a bypass throttle opens. In this manner, a compact construction of the injection valve is achieved.
Further advantageous designs of the inventions are specified in the dependent claims. A chamber through which a connecting rod, which connects a control piston to the nozzle needle, is guided is preferably connected directly to the fuel line which conveys fuel under high pressure. In addition, a leakage line is not connected to the chamber. This largely avoids leakage via the chamber.
An advantageous construction of the injection valve is achieved by the control chamber being bounded by a control piston which is operatively connected to the nozzle needle via a rod. The rod is guided through a chamber in which a needle spring for prestressing the nozzle needle is arranged. In this manner, the control chamber is free from movable parts, so that contamination of the control chamber by components which have been placed in it is prevented. In addition, the control chamber can be of particularly small design, as a result of which the dead volume when activating the nozzle needle is reduced.
The cross section of the control piston is preferably designed to be equal to the cross section of the guided region of the nozzle needle. In this manner, just one guide has to be manufactured, as a result of which the injection valve is cost-effective.
A closing member which is prestressed against a sealing seat by a spring is placed in the valve chamber, said spring likewise being arranged in the valve chamber.